1. Field of the Invention
The present invention relates to a four-wheel-drive vehicle and, more particularly, to a four-wheel-drive vehicle suitable for running at a low running speed on the rough-terrain ground, such as an agricultural working vehicle or a truck.
2. Description of the Related Art
A common four-wheel-drive vehicle for use on the rough-terrain ground is provided with rear axles supported by a swing arm suspension. The swing arm suspension absorbs the vertical movement of the rear wheels caused by the rough surface of the ground. The power unit including an engine, a transmission and such, of the four-wheel-drive vehicle having the swing arm suspension, is mounted on a body frame. The power unit mounted on the body frame is connected to a front reduction gear (or a front differential) for driving the front wheels by a front propeller shaft and to a rear reduction gear (or a rear differential) for driving the rear wheels by a rear propeller shaft.
When the power unit, including the engine, the transmission and such, is mounted on the body frame, the power unit is inevitably disposed at a position in the front half of the vehicle, such as a position under the driver's seat. Consequently, vibrations generated by the engine are transmitted readily to the driver seated on the driver's seat and a narrow space is available around the driver, and therefore ride comfort is deteriorated.
The rear propeller shaft connecting the power unit mounted on the body frame to the rear reduction gear suspended on a swing frame of the swing arm suspension for vertical swinging motion is supported so as to swing vertically together with the swing frame. Consequently, the rear propeller shaft causes power loss greater than that caused by the front propeller shaft. Since load on the rear wheel is large when the four-wheel-drive vehicle is a truck that carries a heavy load on a rear part thereof, the power loss caused by the rear propeller shaft affect adversely to the working economy, such as fuel economy, of the four-wheel-drive vehicle.